1. Field of the Invention
The invention relates to a grinding segment comprising a ceramic part manufactured of grinding ceramic material, at least one fastening hole formed in the ceramic part for fastening the grinding segment by means of a fastening element to the outer surface of a frame of a grinder stone used in defibering of wood.
The invention further relates to a grinder stone for defibering wood, comprising a cylindrical frame, grinding segments made of grinding ceramic material, arranged on the frame's outer circumference and fastened in place by means of a fastening element arranged through at least one fastening hole formed in the segments, and a shaft, around which the grinder stone is arranged to be rotated during grinding.
2. Description of Related Art
Wood is typically ground into fibres by means of grinding machines, where logs are pressed against the surface of a rotating grinder stone. Water is simultaneously sprayed to purify and cool the grinder stone. The stone makes the wood fibre matrix vibrate, whereafter the wood fibres are released from the logs to provide a pulp suspension. Wood can be ground under both a normal pressure and an overpressure.
The frame of the grinder stone is usually made of concrete. A grinding surface consists of separate ceramic grinding segments attached to the frame. The prior art also teaches arrangements, where concrete frames are replaced with a steel drum. Steel is better able to withstand high grinding forces generated during grinding than concrete. Also in a steel drum the grinding segments are attached with suitable bolt mechanisms to the outer circumference of the drum to form the grinding surface of the grinder stone. Different segment fastening mechanisms are disclosed e.g. in FI 67,240. In this reference, a layer of rubber is disposed between a grinding segment and the steel drum to balance the loads between the segment and the frame.
The prior art also teaches different fastening bolt mechanisms, which compensate for varying dimensions resulting from different thermal expansion coefficients of the ceramic segment and the frame of the grinder stone. However, such bolts are rather complicated and therefore also expensive. Furthermore, it is possible to place semi-spherical washers between such bolts and the segment in order that the stresses resulting from the fastening will be distributed more evenly in the segment and the segment will be able to withstand the fastening forces. Failure of a fastening bolt during use results in great damage and an interruption of production. Another problem with the present arrangements is that the fastening forces subject the ceramic segment to compression stress, which can break the segment usually from below the washer. It is thus necessary to restrict the fastening force, which in some cases can lead to insufficient fastening of the segment. During grinding, such a loose segment can start vibrating and will be damaged. The ceramic materials used in grinding are highly wear-resistant and hard, yet brittle. However, in practice grinding segments cannot be manufactured from more resistant ceramic materials, since the ceramic material used in grinding of wood must be of a particular type and sufficiently porous to provide a desired grinding effect. Also, quality characteristics set for mechanical pulp require use of the present grinding ceramic materials.